The field of the disclosure relates generally to electric machines, and more specifically, to detecting the rotor position in switched reluctance machines.
Generally, switched reluctance machines have poles or teeth on both the stator and the rotor. There is a concentrated winding on each of the stator poles, but no windings or permanent magnets on the rotor. Each pair of diametrically opposite stator windings is connected in series or in parallel to form one phase of the switched reluctance motor. Some known switched reluctance machines have more than one phase, e.g., a four pole or six pole switched reluctance machine may have two or three phases, respectively.
In some known switched reluctance machines, such as switched reluctance motors, a controller is generally used to switch current on in each phase in a predetermined sequence to generate a magnetic attraction force between rotor and stator poles that are approaching each other. The controller switches the current off in each phase before the rotor poles nearest the stator poles of that phase rotate past a generally aligned position. This sequential switching on and off of current in the different phases of the switched reluctance motor is responsible for generating the motor torque. If the current is not switched off before the aligned position of the rotor and stator poles in a respective phase, the magnetic attraction force will produce a braking torque in the switched reluctance motor.
Further, in some known switched reluctance motors, the rotor poles are symmetrical with respect to a set of stator poles, resulting in rotor positions where the torque generated by the switching on and off of current in the different phases of the switched reluctance motor is zero. These rotor positions of zero torque make it difficult to start the switched reluctance motor if the stand-still or start-up rotor position corresponds to them. Moreover, in some known switched reluctance motors, because the initial rotor position is not known, the rotor is aligned with the stator by energizing one phase of the motor. This enables the rotor to be placed in a known position to facilitate start-up. However, in some known applications of switched reluctance motors, the motor cannot rotate in a reverse direction to align the rotor poles with the stator.
Generally, for switched reluctance motors to function properly, the current to the phases must be switched on and off in precise synchronism with the rotor position. In some known switched reluctance motors, switching the current on and off based on the rotor position is accomplished using a shaft position sensor. Some known shaft position sensors include Hall effect sensors, which include a transducer that varies its output voltage in response to a magnetic field. However, a disadvantage of using Hall effect sensors is that the elements of the sensor must be very precisely fixed to get an accurate reading of the rotor position. Furthermore, such shaft position sensors are undesirable in small motors because of cost, and in both large and small motors because of space requirements and the vulnerability of the signal wires that must run between the motor and the electronic power converter. In addition, incorporating such position sensors in switched reluctance motors increases the size, cost, and complexity of the motors.